[19.01] Extrasolar Planets and Brown Dwarf Stars

A. P. Boss (DTM/CIW)

The first confirmed detections of extrasolar planets and
brown dwarf stars mark the beginning of a new era for the
discovery of low mass companions to solar-type stars. The
newly-discovered objects appear to be a mixture of gas giant
planets, presumably similar to Jupiter, and brown dwarf
stars, stars with masses less than 80 Jupiters. Brown dwarfs
are too low in mass to undergo hydrogen fusion, making their
detection nearly as difficult as that of extrasolar giant
planets. In most cases, the detection techniques yield only
basic properties for the companions -- their masses (usually
only a lower limit on the mass), their orbital periods and
separations, and their orbital eccentricities. These
properties are sufficient, however, to begin at least a
provisional sorting of the objects into either the giant
planet or brown dwarf category, based in part on theoretical
expectations for the masses and orbital eccentricities of
giant planets and brown dwarfs. Brown dwarf stars are
believed to form on highly eccentric orbits, with masses no
less than about 3 Jupiters, at the same time that the
primary star forms. Giant planets are thought to form after
the primary star has gained most of its mass, within a
protoplanetary disk. Two competing mechanisms exist for
giant planet formation, (1) slow collisional accumulation of
a solid core followed by accretion of disk gas, and (2)
rapid gravitational instability of the disk gas followed by
sedimentation of dust grains to form a core. Both mechanisms
have a long list of advantages and disadvantages, making it
hard to predict theoretically the dominant mechanism.
However, a definitive observational test exists for
determining which mechanism predominantly forms giant
planets. If the youngest stars (less than 1 Myr old) already
have giant planets, then the disk instability mechanism must
be responsible, because the core accretion mechanism
evidently requires greater than 1 Myr to form a planet of
Jupiter-mass.